if you give the prob="1" than you make sure that item will always drop into that lootcontainer, every number below that is a percentage number of a rolling drop chance....
I think this is wrong. I'm not an expert myself and have not tested this conclusively, but look at the following container:
<lootgroup name="electricalDevices">
<item name="resourceElectricParts" count="5,10" prob="1"/>
<item name="resourceElectricParts" count="3,10" prob="0.5"/>
<item name="generatorbank" prob="1"/>
<item name="batterybank" prob="0.7"/>
<item name="pressureplate" count="1,5" prob="1.0"/>
<item name="ceilingLight01_player" count="1,6" prob="1.0"/>
<item name="electrictimerrelay" count="1,3" prob="1.0"/>
<item name="electricwirerelay" count="5,15" prob="1.0"/>
<item name="motionsensor" count="1,2" prob="1.0"/>
<item name="speaker" count="1,2" prob="1.0"/>
<item name="switch" count="3,9" prob="1.0"/>
<item group="schematicsElectrical" count="1" prob="1.0"/>
</lootgroup>
<lootgroup name="workingStiffsSub" count="1,2">
<item group="groupToolsTiered" prob="6"/>
<item name="resourceRepairKit" count="1,4"/>
<item name="smallEngine" prob="0.3"/>
<item group="workstationToolsScaled" prob="2"/>
<item group="automotive" prob="1"/>
<item name="resourceForgedIron" count="12,20" prob=".5"/> <!-- together 3% per crate -->
<item name="resourceForgedSteel" count="8,14" prob=".5"/>
<item name="resourceScrapPolymers" count="35,50" prob=".4"/>
<item group="electricalDevices" prob="1"/>
<item name="resourceMechanicalParts" count="4,10" prob="1"/>
<item name="meleeToolPaintTool" prob="0.4"/>
<item name="meleeToolWireTool"/>
<item name="resourceLockPick" count="1,4"/>
<item group="toolParts"/>
</lootgroup>
<lootgroup name="workingStiffsBox" count="all">
<item group="groupToolsTiered"/>
<item group="workingStiffsSub"/>
</lootgroup>
If you were right and "1" would be certainty, how could there ever be a pressureplate found in the lootgroup electricalDevices ? It would always have a resourceElectricParts in it (if the selection is done in order) or a workingStiffsBox would always have resourceElectricParts AND a generatorbank in it, obviously that isn't the case in game.
A few people at least including me believe that the probabilities are added up and each number is then divided by this to get at the percentage. In other words the probabilities are normalized so they add up to 100%. And yes, that would mean "1" would be the same as "1.0" and have no special meaning.
So electricalDevices probabilities add up to 11.2. The probability to find a generatorbank in it would be 1/11.2 = 0.08 aka 8%. A batterbank would have .7/11.2= 0.06 aka 6% probability to be found in the lootgroup electricalDevices.